In 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a mobile communication system, the introduction of Device to Device (D2D) proximity service (D2D ProSe) is discussed as a new function after Release 12 (see non patent document 1).
In D2D communication, which is an example of the D2D proximity service, a plurality of proximal user terminals perform direct device-to-device communication without passing through a network. On the other hand, in cellular communication, which is normal communication in a mobile communication system, a user terminal performs communication through a network.
Further, as an example of an operation in the D2D proximity service, the user terminal discovers another user terminal by a discovery signal (Discovery signal or Discoverable signal) used for discovery of a partner terminal (proximal terminal) in the D2D communication.